(1) Field of the Invention
The present invention relates to an image control apparatus, and more particularly to an image control apparatus for controlling a process of coding an image signal.
(2) Description of the Related Art
Image coding processes such as MPEG standardized by ISO/IEC and H.262 standardized by ITU-T introduce bidirectional predictive coded images (B pictures) in addition to intraframe predictive images (I pictures) and interframe forward predictive images (P pictures).
Data in each of those pictures is divided into small blocks, and processed in each of the blocks. Bidirectional prediction is a process of predicting an image frame using image frames in the past and future, and uses I pictures and P pictures for prediction.
A resultant encoded data stream comprises an I picture at its start and some B pictures inserted between I and P pictures or between P pictures (IBBPBBPB . . . ). A collection of pictures including an I picture at its start is called GOP (group of pictures).
In the conventional GOP, the number of repeated B pictures is generally 2. Such a value is suitable for average picture patterns, and other values than 2 may be appropriate for some picture patterns. Accordingly, no optimum coding process has heretofore been carried out in the art.
For example, as an input image is closer to a still image, a suitable value for B pictures increases. Therefore, the coding efficiency increases by detecting the motion of an input image and changing the number of repeated B pictures depending on the extent of the detected motion.
When a scene change occurs, the image quality is often improved by performing the intraframe coding process. Heretofore, it has been customary to carry out the intraframe coding process with fixed timing irrespective of a scene change. The coding efficiency has been poor because the intraframe coding process has not been carried out appropriately depending on the state of input images.